M49 Oliver Archut alt version

[Spencerleehorton]

10 R and 1000uf put in and still have hum?

 

[Spencerleehorton]

the hum is gone now, i had the bridge rectifiers gnd linked and then one gnd wire to chassis, i have now seperated them and given each rectifier its own gnd wire to chassis and no hum!! happy days

 

[Murdock]

If there are no separate wires for plate 0v and heater 0v going to the mic, would it still be better to keep the two separated as long as possible (until the connector inside the PSU)?

 

[Spencerleehorton]

well it seems to not need that, just make sure each rectifier has a seperate gnd to chassis, but someone will probably comment that it will make a difference!

thanks Ruud and khron for your help, got there in the end.

5840 has been commented on as being quite microphonic, i have put some tape inside the mic body to stop ringing and has helped so much, much better clarity now as there was some low end ringing coming from the body, i might put on a couple of the rubber rings for tubes to help the tube as well, see if that makes a difference.

 

[Murdock]

Could it be, that the only problem was to connect both together only at the rectifiers and going from there to the chassis?
I assume if you would connect them at the output or having a groundplane and going from the output to the chassis it would be better.
I’ve read you better should connect 0v to chassis at the output close to the connector.

 

[HerbertR]

So, I've now spent lots of hours optimizing the noise level. Since I built two largely identical M49 Archut Mics, I could easily compare all modifications directly. The last tubes, QC tested 5840 tubes from Raytheon from Royer L (blue ...printed), basically sound good. On the one hand, the gain is more than about 1-2dB louder as from all other tested tubes, but unfortunately the basic sound is much harder, sibilance is much more pronounced, for example compared with the 5840 tubes from Thompson (white print), which I really like in terms of the basic character. Round, powerful, silbance balanced, smoother, and slight airy, sooo noble. The S/N ratio is a bit better with the Royer tubes. But what I didn't like at all is that the sound is different on all three Royer tubes I bought, too inhomogeneous for my taste to pass as matched. Also when comparing the Noisefloorspectrum, there are huge differences. The Raytheon (label red printed) actually falls through from the noise floor, much too much, harder character, the Raytheon Philipps Jan (green printed) unfortunately had an unusable tube with quiet, discontinuous radio chirping. So I could not get a pair. At least the working one sounded well, but not as good as the Thompson ( white)
You can see testing this Tubes takes time. They look all different, defnitly all from different manufacture companys....Always desoldering, burn in at least over one day, comparing...
I separated the heater grounding from the tube and connected it directly to the potential screw in the PSU Case. That helped a lot and so I was able to bring the noise floor down to a good level. Now I keep up with the same S/N of my old C3000 on an equal footing. Not the lowest level, but works really good now.....
Personally I would rather advise against building an OA M49 with 5840 tubes. As good as the tube mic sounds in the end, it is a "crux" with finding the necessary, low-noise tubes, which then also have very different sound images..... without the AC701 its a experimental build, more lottery in my opinion...

 

[rock soderstrom]

You can see testing this Tubes takes time.

....Always desoldering, burn in at least over one day, comparing...

This is really a disadvantage of tubes that are soldered in directly...It sucks if you don't have a separate test jig where you can temporarily clip the wire ends.

 

[ubxf]

This is really a disadvantage of tubes that are soldered in directly...It sucks if you don't have a separate test jig where you can temporarily clip the wire ends.

There are some sockets for sub mini tubes and some people also use test socket for IC to avoid soldering

 

[rock soderstrom]

There are some sockets for sub mini tubes and some people also use test socket for IC to avoid soldering

Yes, I know. I tried the sockets, mine were not exactly high quality. You can make yourself new problems with it in the high impedance region.

For me the best way is to build a separate test rig with brackets....

or to use 7pin tubes like the 5654 if they fit in the mic body

 

[HerbertR]

I don't know how many hours I've spent optimizing this M49 Oliver Archut circuit to make both of my builds less noisy. In the end I soldered the original M49c circuit with cathode self bias on a circuit board and turret pins. I spent the mics a huge Cinemag 6511 7:1 output transformers, tested low noise Phillips Raytheon 5840 tubes from Royer Labs with a 10uF cathode cap, and also set the C4 "Abgleichen" coupling capacitor from 8pF to a 2pF mica caps. And lo and behold - the mics are finally super quiet, the higher noise ratio is history. The reduction of the coupling capacitor C4 once again gives the signal a decent presence, the output level increases over 6dB. Insanely awesome tube microphones now.....Hallelujah.....
Never give up, and goodbye M49 OA Circuit.....